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<title>SSL Encrypting syslog with stunnel</title><meta name="KEYWORDS" content="syslog encryption, rsyslog, stunnel, secure syslog, tcp, reliable, howto, ssl"></head><body>
<h1>SSL Encrypting Syslog with Stunnel</h1>
		<p><small><i>Written by
		<a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer 
		Gerhards</a> (2005-07-22)</i></small></p>
<h2>Abstract</h2>
<p><i><b>In this paper, I describe how to encrypt <a href="http://www.monitorware.com/en/topics/syslog/">syslog</a>
messages on the network.</b> Encryption
is vital to keep the confidiental content of syslog messages secure. I describe the overall
approach and provide an HOWTO do it with the help of 
<a href="http://www.rsyslog.com">rsyslogd</a> and <a href="http://www.stunnel.org">stunnel</a>.</i></p><p><span style="font-weight: bold;">Please note that starting with rsyslog 3.19.0, </span><a style="font-weight: bold;" href="rsyslog_tls.html">rsyslog provides native TLS/SSL encryption</a><span style="font-weight: bold;"> <span style="font-style: italic;">without</span> the need of stunnel. </span>I
strongly recomend to use that feature instead of stunnel. The stunnel
documentation here is mostly provided for backwards compatibility. New
deployments are advised to use native TLS mode.<i></i></p>
<h2>Background</h2>
<p><b>Syslog is a 
clear-text protocol. That means anyone with a sniffer can have 
a peek at your data.</b> In some environments, this is no problem at all. In 
others, it is a huge setback, probably even preventing deployment of syslog 
solutions. Thankfully, there is an easy way to encrypt syslog communication. I 
will describe one approach in this paper.</p>
<p>The most straightforward solution would be that the syslogd itself encrypts 
messages. Unfortuantely, encryption is only standardized in
<a href="http://www.monitorware.com/Common/en/glossary/rfc3195.php">RFC 3195</a>. But there 
is currently no syslogd that implements RFC 3195's encryption features, 
so this route leads to nothing. Another approach would be to use vendor- or 
project-specific syslog extensions. There are a few around, but the problem here 
is that they have compatibility issues. However, there is one surprisingly easy 
and interoperable solution: though not standardized, many vendors and projects 
implement plain tcp syslog. In a nutshell, plain tcp syslog is a mode where 
standard syslog messages are transmitted via tcp and records are separated by 
newline characters. This mode is supported by all major syslogd's (both on Linux/Unix 
and Windows) as well as log sources (for example,
<a href="http://www.eventreporter.com/en/">EventReporter</a> for Windows 
Event Log forwarding). Plain tcp syslog offers reliability, but it does not 
offer encryption in itself. However, since it operates on a tcp stream, it is now easy 
to add encryption. There are various ways to do that. In this paper, I will 
describe how it is done with stunnel (an
other alternative would be <a href="http://en.wikipedia.org/wiki/IPSec">IPSec</a>, for example).</p>
<p>Stunnel is open source and it is available both for Unix/Linux and Windows. 
It provides a way to 
			use ssl communication for any non-ssl aware client and server - in this case,
		our syslogd.</p>
		<p>Stunnel works much like a wrapper. Both on the client and on the server machine, 
			tunnel portals are created. The non-ssl aware client and server software is 
			configured to not directly talk to the remote partner, but to the local 
			(s)tunnel portal instead. Stunnel, in turn, takes the data received from the 
			client, encrypts it via ssl, sends it to the remote tunnel portal and that 
			remote portal sends it to the recipient process on the remote machine. The 
			transfer to the portals is done via unencrypted communication. As such, 
			it is vital that 
			the portal and the respective program that is talking to it are on the same 
			machine, otherwise data would travel partly unencrypted. Tunneling, as done by stunnel, 
		requires connection oriented communication. This is why you need to use 
		tcp-based syslog. As a side-note, you can also encrypt a plain-text RFC 
		3195 session via stunnel, though this definitely is not what the 
		protocol designers had on their mind ;)</p>
<p>In the rest of this document, I assume that you use rsyslog on both the 
client and the server. For the samples, I use <a href="http://www.debian.org/">Debian</a>.
Interestingly, there are 
some annoying differences between stunnel implementations. For example, on 
Debian a comment line starts with a semicolon (';'). On 
<a href="http://www.redhat.com">Red Hat</a>, it starts with 
a hash sign ('#'). So you need to watch out for subtle issues when setting up 
your system.</p>
<h2>Overall System Setup</h2>
<p>In ths paper, I assume two machines, one named "client" and the other named "server". 
It is obvious that, in practice, you will probably have multiple clients but 
only one server. Syslog traffic shall be transmitted via stunnel over the 
network. Port 60514 is to be used for that purpose. The machines are set up as 
follows:</p>
<p><b>Client</b></p>
<ul>
	<li>rsyslog forwards&nbsp; message to stunnel local portal at port 61514</li>
	<li>local stunnel forwards data via the network to port 60514 to its remote 
	peer</li>
</ul>
<p><b>Server</b></p>
<ul>
	<li>stunnel listens on port 60514 to connections from its client peers</li>
	<li>all connections are forwarded to the locally-running rsyslog listening 
	at port 61514</li>
</ul>
<h2>Setting up the system</h2>
<p>For Debian, you need the "stunnel4" package. The "stunnel" package is the 
older 3.x release, which will not support the configuration I describe below. 
Other distributions might have other names. For example, on Red Hat it is just "stunnel". 
Make sure that you install the appropriate package on both the client and the 
server. It is also a good idea to check if there are updates for either stunnel 
or openssl (which stunnel uses) - there are often security fixes available and 
often the latest fixes are not included in the default package.</p>
<p>In my sample setup, I use only the bare minimum of options. For example, I do 
not make the server check client cerficiates. Also, I do not talk much about 
certificates at all. If you intend to really secure your system, you should 
probably learn about certificates and how to manage and deploy them. This is 
beyond the scope of this paper. For additional information,
<a href="http://www.stunnel.org/faq/certs.html">
http://www.stunnel.org/faq/certs.html</a> is a good starting point.</p>
<p>You also need to install rsyslogd on both machines. Do this before starting 
with the configuration. You should also familarize yourself with its 
configuration file syntax, so that you know which actions you can trigger with 
it. Rsyslogd can work as a drop-in replacement for stock
<a href="http://www.infodrom.org/projects/sysklogd/">sysklogd</a>. So if you know 
the standard syslog.conf syntax, you do not need to learn any more to follow 
this paper.</p>
<h3>Server Setup</h3>
<p>At the server, you need to have a digital certificate. That certificate 
enables SSL operation, as it provides the necessary crypto keys being used to 
secure the connection. Many versions of stunnel come with a default certificate, 
often found in /etc/stunnel/stunnel.pem. If you have it, it is good for testing 
only. If you use it in production, it is very easy to break into your secure 
channel as everybody is able to get hold of your private key. I didn't find an 
stunnel.pem on my Debian machine. I guess the Debian folks removed it because of 
its insecurity.</p>
<p>You can create your own certificate with a simple openssl tool - you need to 
do it if you have none and I highly recommend to create one in any case. To 
create it, cd to /etc/stunnel and type:</p>
<p></p><blockquote><code>openssl req -new -x509 -days 3650 -nodes -out 
stunnel.pem -keyout stunnel.pem</code></blockquote><p></p>
<p>That command will ask you a number of questions. Provide some answer for 
them. If you are unsure, read
<a href="http://www.stunnel.org/faq/certs.html">
http://www.stunnel.org/faq/certs.html</a>. After the command has finished, you 
should have a usable stunnel.pem in your working directory.</p>
<p>Next is to create a configuration file for stunnel. It will direct stunnel 
what to do. You can used the following basic file:</p>
<p></p><blockquote><code></code><pre>; Certificate/key is needed in server mode<br>cert = /etc/stunnel/stunnel.pem<br><br><i>; Some debugging stuff useful for troubleshooting<br>debug = 7<br>foreground=yes</i>

[ssyslog]
accept  = 60514
connect = 61514</pre>
</blockquote><p></p>
<p>Save this file to e.g. /etc/stunnel/syslog-server.conf. Please note that the 
settings in <i>italics</i> are for debugging only. They run stunnel 
with a lot of debug information in the foreground. This is very valuable while 
you setup the system - and very useless once everything works well. So be sure 
to remove these lines when going to production.</p>
<p>Finally, you need to start the stunnel daemon. Under Debian, this is done via 
"stunnel /etc/stunnel/syslog.server.conf". If you have enabled the debug 
settings, you will immediately see a lot of nice messages.</p>
<p>Now you have stunnel running, but it obviously unable to talk to rsyslog - 
because it is not yet running. If not already done, configure it so that it does 
everything you want. If in doubt, you can simply copy /etc/syslog.conf to /etc/rsyslog.conf 
and you probably have what you want. The really important thing in rsyslogd 
configuration is that you must make it listen to tcp port 61514 (remember: this 
is where stunnel send the messages to). Thankfully, this is easy to achive: just 
add "-t 61514" to the rsyslogd startup options in your system startup script. 
After done so, start (or restart) rsyslogd.</p>
<p>The server should now be fully operational.</p>
<h3>Client Setup</h3>
<p>The client setup is simpler. Most importantly, you do not need a certificate 
(of course, you can use one if you would like to authenticate the client, but 
this is beyond the scope of this paper). So the basic thing you need to do is 
create the stunnel configuration file.</p>
<p></p><blockquote><code></code><pre><i>; Some debugging stuff useful for troubleshooting<br>debug = 7<br>foreground=yes</i>

<b>client=yes</b>

[ssyslog]
accept  = 127.0.0.1:61514
connect = <font color="#ff0000">192.0.2.1</font>:60514<br></pre>
</blockquote><p></p>
<p>Again, the text in <i>italics</i> is for debugging purposes only. I suggest 
you leave it in during your initial testing and then remove it. The most 
important difference to the server configuration outlined above is the "client=yes" 
directive. It is what makes this stunnel behave like a client. The accept 
directive binds stunnel only to the local host, so that it is protected from 
receiving messages from the network (somebody might fake to be the local sender). 
The address "192.0.2.1" is the address of the server machine. You must change it 
to match your configuration. Save this file to /etc/stunnel/syslog-client.conf.</p>
<p>Then, start stunnel via "stunnel4 /etc/stunnel/syslog-client.conf".&nbsp; Now 
you should see some startup messages. If no errors appear, you have a running 
client stunnel instance.</p>
<p>Finally, you need to tell rsyslogd to send data to the remote host. In stock 
syslogd, you do this via the "@host" forwarding directive. The same works with 
rsyslog, but it suppports extensions to use tcp. Add the following line to your 
/etc/rsyslog.conf:</p>
<p></p><blockquote><code></code><pre>*.*      @<font color="#ff0000">@</font>127.0.0.1:61514<br></pre>
</blockquote><i><p></p>

</i>

<p>Please note the double at-sign (@@). This is no typo. It tells rsyslog to use 
tcp instead of udp delivery. In this sample, all messages are forwarded to the 
remote host. Obviously, you may want to limit this via the usual rsyslog.conf 
settings (if in doubt, use man rsyslog.con).</p>
<p>You do not need to add any special startup settings to rsyslog on the client. 
Start or restart rsyslog so that the new configuration setting takes place.</p>
<h3>Done</h3>
<p>After following these steps, you should have a working secure syslog 
forwarding system. To verify, you can type "logger test" or a similar smart 
command on the client. It should show up in the respective server log file. If 
you dig out you sniffer, you should see that the traffic on the wire is actually 
protected. In the configuration use above, the two stunnel endpoints should be 
quite chatty, so that you can follow the action going on on your system.</p>
<p>If you have only basic security needs, you can probably just remove the debug 
settings and take the rest of the configuration to production. If you are 
security-sensitve, you should have a look at the various stunnel settings that 
help you further secure the system.</p>
<h2>Preventing Systems from talking directly to the rsyslog Server</h2>
<p>It is possible that remote systems (or attackers) talk to the rsyslog server 
by directly connecting to its port 61514. Currently (July of 2005), rsyslog does 
not offer the ability to bind to the local host, only. This feature is planned, 
but as long as it is missing, rsyslog must be protected via a firewall. This can 
easily be done via e.g iptables. Just be sure not to forget it.</p>
<h2>Conclusion</h2>
<p>With minumal effort, you can set up a secure logging infrastructure employing 
ssl encrypted syslog message transmission. As a side note, you also have the 
benefit of reliable tcp delivery which is far less prone to message loss than 
udp.</p>
<h3>Feedback requested</h3>
<p>I would appreciate feedback on this tutorial. If you have additional ideas, 
comments or find bugs (I *do* bugs - no way... ;)), please
<a href="mailto:rgerhards@adiscon.com">let me know</a>.</p>
<h2>Revision History</h2>
<ul>
	<li>2005-07-22 * 
	<a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer Gerhards</a> * Initial Version created</li>
	<li>2005-07-26 *
	<a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer Gerhards</a> * Some text brush-up, hyperlinks added</li>
	<li>2005-08-03 *
	<a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer Gerhards</a> 
	* license added</li><li>2008-05-05 *	<a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer Gerhards</a> 
	* updated to reflect native TLS capability of rsyslog 3.19.0 and above</li>
</ul>
<h2>Copyright</h2>
<p>Copyright (c)  2008 <a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer Gerhards</a> and
<a href="http://www.adiscon.com/en/">Adiscon</a>.</p>
<p>      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.2
      or any later version published by the Free Software Foundation;
      with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
	 Texts.  A copy of the license can be viewed at
<a href="http://www.gnu.org/copyleft/fdl.html">
http://www.gnu.org/copyleft/fdl.html</a>.</p>
<p>[<a href="manual.html">manual index</a>]
[<a href="rsyslog_conf.html">rsyslog.conf</a>]
[<a href="http://www.rsyslog.com/">rsyslog site</a>]</p>
<p><font size="2">This documentation is part of the
<a href="http://www.rsyslog.com/">rsyslog</a> project.<br>
Copyright &copy; 2008 by <a href="http://www.gerhards.net/rainer">Rainer Gerhards</a> and
<a href="http://www.adiscon.com/">Adiscon</a>. Released under the GNU GPL
version 2 or higher.</font></p>

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